The turbine section of a gas turbine engine contains a rotor shaft and one or more turbine stages, each having a turbine disk (or rotor) mounted or otherwise carried by the shaft and turbine blades mounted to and radially extending from the periphery of the disk. A turbine assembly typically generates rotating shaft power by expanding hot compressed gas produced by combustion of a fuel. Gas turbine buckets or blades generally have an airfoil shape designed to convert the thermal and kinetic energy of the flow path gases into mechanical rotation of the rotor.
Within a turbine engine, a shroud is a ring of material surrounding the rotating blades. Turbine performance and efficiency may be enhanced by reducing the space between the tip of the rotating blade and the stationary shroud to limit the flow of air over or around the top of the blade that would otherwise bypass the blade. For example, a blade may be configured so that its tip fits close to the shroud during engine operation. Thus, generating and maintaining an efficient tip clearance is particularly desired for efficiency purposes.
During engine operation, the blade tips can sometime rub against the shroud, thereby increasing the gap and resulting in a loss of efficiency, or in some cases, damaging or destroying the blade set. For CMC shrouds, damage to a metal blade is even more likely, since the silicon carbide material is significantly harder than the Nickel-based superalloys. For a CMC shroud, and Environmental Barrier Coating is also required for successful performance/survival of a the part due to material loss from high temperature steam recession. Thus, if rub from a blade tip takes off the coating, the CMC shroud is left susceptible to high temperature steam recession.
In order to reduce the risk of coating loss, an abradable layer is deposited on top of the EBC to protect it from blade rub. Generally, the abradable layer is a series of ceramic ridges that break away upon contact with a rotating blade tip. The ceramic material is typically made out of the same ceramic material as one of the environmental barrier layers, for example, rare earth disilicate or barium strontium aluminosilicate (BSAS).
The patterned ridges on the surface of the shroud, however, lead to aerodynamic losses to the efficiency of the engine. Additionally, it is difficult to produce a thick, continuous layer of the EBC materials (>20 mils) without spallation. Furthermore, a continuous ceramic layer is typically quite hard, and does not abrade but rather damages the rotating blades.
Thus, an improved abradable coating for a CMC component, particularly a shroud, is desirable in the art.